Scheelite (w. chalcocite, quartz and siderite)

TSNB615
Specimen
Acosta, C.Amo, C.Casanova, I.Schumer, B.
Exhibit 1. Scheelite (w. chalcocite, quartz and siderite); 42 mm.
Exhibit 1. Scheelite (w. chalcocite, quartz and siderite); 42 mm.
Image Credit: Malcolm Southwood
Exhibit 2. 20 mm field of view.
Exhibit 2. 20 mm field of view.
Image Credit: Malcolm Southwood
Exhibit 3. 5 mm field of view.
Exhibit 3. 5 mm field of view.
Image Credit: Malcolm Southwood
Exhibit 4. 32 mm field of view.
Exhibit 4. 32 mm field of view.
Image Credit: Malcolm Southwood
Thumbnail by 1Thumbnail by 2Thumbnail by 3Thumbnail by 4

Specimen Title

Scheelite (w. chalcocite, quartz and siderite)

Associated Minerals

chalcocite; fraipontite (?); quartz; siderite; undetermined

Principal Mineral

Scheelite

Size

Miniature; 42mm

Location in the Mine

Third oxidation zone; 46 Level

Provenance

Schumer, B.; Acosta, C.; Casanova, I.; Amo, C.

Collection

Southwood, M.; MS2017.026

Entry Number

Specimen; TSNB615

Description

A shard of sulphide matrix encrusted with slender prismatic quartz crystals (to 6mm), supplanted by well-formed crystals of silver-grey chalcocite (to 9 mm), aggregates of discoidal orange-yellow siderite crystals, and sparse bi-pyramidal crystals of translucent, honey-brown scheelite (to circa 1 mm). The scheelite fluoresces green in short wavelength ultraviolet. The entire specimen is shown in Exhibit 1, while the scheelite crystals are better seen in exhibits 2 and 3. In Exhibit 3 a little snow-white, microcrystalline fraipontite (?) is visible at upper left of centre and an undetermined sulphide phase is present between the two prominent chalcocite crystals. Exhibit 4 reveals the habit and colour variation - zoning - of the siderite crystals.

Scheelite was confirmed by EDS at the University of Nevada. An almost identical specimen at Harvard, MGMH 2022.4.10385, has been studed in greater detail and the scheelite found to be Mo-rich, with a molar ratio of roughly 3:2 of tungstate to molybdate (Mark Feinglos, pers.comm. to M. Southwood, 2015). It therefore lies towards the middle of the scheelite - powellite series. Fraipontite was also confirmed on the MGMH specimen by XRD.

This specimen was purchased from Chris Amo in 2016. Amo acquired it from Isaias Casanova and it was formerly in the collections of Carl Acosta and Ben Schumer. The paragenesis has been described in detail by Gebhard (1999) as a unique find from 46 Level in the third oxidation zone.